Canada thistle (Cirsium arvense) is probably near the top of the list for just about any gardener and farmers too for that matter. It is a tough perennial broadleaf weed that is easily established in your garden by the seeds that are carried in by our nearly continuous winds, thanks to the little tuft of hairs that is attached to one end of each seed.
|Column by David Graper, SDSU Extension Horticulture Specialist & South Dakota Master Gardener Interim Program Coordinator
Once those seedling thistles start growing, they quickly establish a deep root system and begin growing underground stems (rhizomes) that have the capacity to produce numerous new plants, one from each node or bud along those stems. It is almost impossible to completely pull out an established thistle plant because it is probably growing up from a rhizome and will just break off when you give it a tug. That will just stimulate the growth of new plants from other nodes along the stem. All of these characteristics combine to make this a noxious weed in 37 states.
Canada thistle is a perennial coming back year after year from its extensive network of roots and rhizomes. It can quickly form large patches of plants that continue to spread farther if they are allowed to flower and go to seed.
What sets Canada Thistle Apart
They can be distinguished from many of the other common thistles by several key factors. First of all, many thistles are biennial, producing a low-growing rosette of foliage then only flowering the second year.
Canada thistles can bloom the first year from seed and then bloom each year after that from the new stems that emerge from the soil each spring or throughout the growing season. The stems are hollow and spineless but the leaves have short spines along their wavy margins. The undersides of the leaves are green, not white, like they are on some other species of thistles.
The flowers are also relatively small, borne in branched clusters at the top of the stems, compared to the much larger, usually single flower heads of other thistles. They are also unisexual, either male or female, requiring cross-pollination in order for seed to be produced.
Control of Canada thistle can be quite difficult, depending on where they are growing.
If a new garden spot is being prepared, achieving control of perennial weeds like thistles before planting is a great way to start.
Non-selective herbicides like glyphosate work fairly well if applied when the thistles have grown several inches tall to provide plenty of leaf surface for the uptake of the herbicide.
Cultivation is another option but just one cultivation will generally result in a huge flush of new plants developing from all of the severed rhizomes. Cultivation will need to be repeated several times over the course of the growing season and perhaps for several years to eventually deplete stored food reserves in the roots and rhizomes. Repeated mowing can also be of benefit, but it will take quite some time to actually kill well established colonies of plants. Smothering with black plastic or some other material is also possible but will take time to be effective.
Canada thistle infested lawns are also quite common. Many of the broadleaf weed control herbicides can provide good suppression and eventually control if applied in the fall of the year. Repeat applications in subsequent years will likely be needed. In any case, if your lawn or garden is near thistle infested fields or road ditches, you can bet that they will be back.
It is the responsibility of everyone to do what they can to control Canada thistles that are growing on their land.